1. Field of the Invention
The present invention relates to the field of semiconductor devices and, more particularly, to photoresists and to a method of stabilizing 193 nm photoresists by implanting ions therein.
2. Discussion of the Technology
The production of photoresists is well known in the art as exemplified in, for example, U.S. Pat. Nos. 3,666,473; 4,115,128 and 4,173,470. Generally, these photoresists contain aqueous alkali soluble polyvinyl phenol or phenol formaldehyde novolak resins together with light sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent and are applied as a thin film coating to a substrate suitable for the particular application desired. The resin component of photoresist formulations is soluble in an aqueous alkaline solution, but the photosensitizer is not. Continuing with the process, the photoresist may then be subjected to a baking step often called a “soft-bake.” Next, the photoresist is selectively exposed to a form of radiation such as ultraviolet (UV) light, electrons or x-rays in order to create a latent image on the resist, optionally followed by a post-exposure bake. Upon the imagewise exposure of the coated substrate to radiation, the exposed areas of the coating are rendered more soluble than the unexposed areas. The difference in solubility causes the exposed areas of the photoresist coating to be dissolved when the substrate is subsequently immersed in a developing solution, while the unexposed areas are substantially unaffected, thereby producing a positive image on the substrate. Areas of the substrate from which the photoresist has been removed can be subjected to a variety of subtractive (i.e., etching) or additive (i.e., ion implantation) processes that transfer the pattern onto the substrate surface. Frequently, the etching involves a plasma etching against which the resist coating must be sufficiently stable. Because photoresist coating protects the covered areas of the substrate from the etchant, the etchant is only able to etch the uncovered areas of the substrate. A pattern, therefore, can be created on the substrate. The pattern on the substrate corresponds to the pattern of the mask or template that was used to create selective exposure patterns on the coated substrate prior to development. Ion implantation is a process in which energetic, charged atoms (or molecules) are directly introduced into a substrate. It is primarily used in VLSI and ultra large scale integration (ULSI) fabrication to add dopant ions into the surface of silicon wafers. The photoresists are used as a masking material so that dopants can be introduced into selected regions (i.e., those not covered by the photoresist).
The photoresist may also subsequently be subjected to one or more post-development steps. For example, a post-development bake is a process in which the resist is subjected to an elevated temperature upon completion of development and prior to etching. Its chief functions are to remove residual solvents, to improve adhesion and to increase the etch resistance of the photoresist. After or during post-development baking, photoresists may be subjected to an additional stabilization process. This stabilization is a photostabilization process typically done using a combination of UV radiation and heat. Photostabilization makes the photoresist less susceptible to erosion and prevents bubbling and blistering in the resist film. These methods, however, are no longer effective for 193 nm resists because of the nature of the resists (i.e., increased instability and sensitivity). For example, the current generation of 193 nm photoresists suffers from severe instability under plasma conditions for various etching processes including high aspect ratio, self-aligned contact and gate etches. Such instability is problematic because it results in surface damage to the photoresist (i.e., pin holes), a loss of photoresist (i.e., low selectivity) and/or critical dimension and formation of striations, which subsequently leads to problems during plug fill. In particular, current ArF 193 nm photoresists are very “soft” in nature and causes raggedness, striation and pin holes during the etch of the underlying wafer layers.
The ability to reproduce very small dimensions is extremely important in the production of large scale integrated circuits on silicon chips and similar components. As the integration degree of semiconductor devices becomes higher, finer photoresist film patterns are required. One way to increase circuit density on such a chip is by increasing the resolution capabilities of the resist.
The optimally obtainable microlithographic resolution is essentially determined by the radiation wavelengths used for selective irradiation. The resolution capacity that can be obtained with conventional deep ultraviolet microlithography, however, has its limits. In order to be able to sufficiently resolve optically small structural elements, wavelengths shorter than deep UV radiation must be utilized. The use of UV radiation has been employed for many applications, particularly radiation with a wavelength of 193 nm. In particular, the radiation of an argon fluoride (ArF) excimer laser, which has a wavelength of 193 nm, is useful for this purpose. The deep UV photoresist materials that are used today, however, are not suitable for 193 nm exposure. Materials based on phenolic resins as a binding agent, particularly novolak resins or polyhydroxystyrene derivatives, have too high an absorption at wavelengths below 200 nm and one cannot image through films of the necessary thickness. This high absorption at 193 nm radiation results in side walls of the developed resist structures that do not form the desired vertical profiles. Rather, they have an oblique angle with the substrate, which causes poor optical resolution characteristics at these short wavelengths.
Chemical amplification resist films have been developed, which have been found to have superior resolution. The chemistry of a 193 nm photoresist is based on polymers such as, but not limited to, acrylates, cyclic olefins with alicyclic groups and hybrids of the aforementioned polymers which lack aromatic rings, which contribute to opacity at 193 nm. It has, therefore, been known to utilize photoresists based on methacrylate resins for the production of microstructures by means of 193 nm radiation.
Chemically amplified resist films, however, have not played a significant role in the fine pattern process using deep UV because they lack sufficient etch resistance, thermal stability, post exposure delay stability and processing latitude. While such photoresists are sufficiently transparent for 193 nm radiation, they do not have the etching stability for plasma etching that is customary for resists based on phenolic resins. A typical chemical amplification photoresist film comprises a polymer, a photoacid generator and other optional additives. The polymer is required to be soluble in the chosen developer solution and to have high thermal stability and low absorbency to the 193 nm exposure wavelength in addition to having excellent etch resistance. Because resists containing aromatic compounds show high absorbency to ArF (193 nm), while non-aromatic matrix resins have a poor etch resistance, these contrasting weak points are factors retarding the development of excellent photoresist films for ArF lithography.
To improve etch resistance, several approaches for increasing polymer deposition during the etch have been tested. These methods, however, run the risk of tapering profile and causing etch stop.
U.S. Pat. No. 6,319,655 B1 to Wong et al. describes aprocess for increasing the etch resistance of photoresists by exposing the photoresists to sufficient electron beam radiation. Nonetheless, the difficulties addressed by the prior art are still present.
A need, therefore, exists for stable 193 nm photoresists and a method of making the same.